The use of ferric chloride as a catalyst in chlorination and fluoride-chloride exchange reactions involving halopyridines is well known and ferric chloride is a common component of reaction mixtures obtained in the manufacture of halopyridines. For example, ferric chloride is a catalyst for the preparation of 2,3,5,6-tetrachloropyridine by chlorination of 2,6-dichloro-pyridine (U.S. Pat. No. 3,538,100), 3,5-dichloro-pyridine (U.S. Pat. No. 4,225,718), or 2-chloro-6-(trichloromethyl)pyridine (U.S. Pat. No. 4,256,894); the preparation of 2,3-dichloro-6-(trichloromethyl)-pyridine by chlorination of 2-chloro-6-(trichloromethyl)-pyridine (U.S. Pat. No. 4,256,894); the preparation of 3,5-dichloro-2,4,6-trifluoropyridine by halogen exchange of pentachloropyridine (U.S. Pat. No. 4,542,221) and the preparation of 2,3-dichloro-5-(trifluoromethyl)-pyridine by halogen exchange of 2,3-dichloro-5-(tri-chloromethyl)pyridine (EPO Application No. 83307212.7) or 2-fluoro-3-chloro-5-(trifluoromethyl)pyridine (U.S. Pat. No. 4,546,192). Additionally, ferric chloride is commonly present as an undesirable corrosion product in such reactions. This ferric chloride must generally be removed from the halopyridine in a purification procedure before the halopyridine can be sold or utilized as an intermediate for the preparation of other useful products.
Ferric chloride is conventionally removed from halopyridine and other halogenated compound mixtures containing it by distillation, which leaves the ferric chloride as a non-distillable residue, or by dissolving the halogenated compound in a chlorinated hydrocarbon solvent, which leaves the ferric chloride as a solid that can be separated by filtration. Neither of these methods is very efficient and their utility in commercial operations is limited. This is because halopyridines appear to bond to ferric chloride in some manner and this bonding results not only in a high apparent solubility of ferric chloride in halopyridines, but also in a difficulty in separating them. Thus, even though it is possible and often desirable to isolate the bulk of a halopyridine from a mixture by distillation, distillation alone is not efficient because after completion of the distillation much of the halopyridine remains in the residue with the ferric chloride and cannot be separated from the ferric chloride by further distillation. Heating to higher temperatures causes decomposition of halopyridines and is therefore wasteful and dangerous. Extraction of ferric chloride contaminated halopyridines with chlorinated hydrocarbon solvents is also not efficient as that portion of the halopyridine bound to ferric chloride is not extracted. Additionally, the chlorinated hydrocarbon solvent employed must be removed from the halopyridine in a subsequent operation. This situation and the high value of many halopyridines makes it desirable to have improved methods of separating ferric chloride from halopyridines.